Go to the BlackBerry Enterprise Server Management console and open it. Now right-click on the BlackBerry Smartphone user account whose password you want to change. Go to IT admin, then to set password and Lock. In this window type your new password into the new password box and new password again box and then click OK
After around 30 min you will receive the IT policy and when you accept it your password is changed.
Now, you can use your new password for your phone!